Mid- to late-Holocene reservoir-age variability and isotope-based palaeoenvironmental reconstruction in the Limfjord, Denmark

Abstract

Palaeoenvironmental and 14 C reservoir age variability in the Limfjord, a sound through northern Jutland, Denmark, was investigated for the period 7300 to 1300 cal yr BP. Shells and bulk sediment samples from a core from a former inlet, Kilen, were analysed by radiocarbon dating and stable isotope (C/N) measurements. A strong correlation between the C/N ratios and δ 13 C values verifies that these are good carbon source indicators and thus allow environmental reconstructions. Furthermore, δ 13 C values are correlated with salinity in the photic zone, inferred quantitatively from diatom assemblages. They are therefore used to differentiate between brackish and marine palaeo-conditions. 14 C reservoir ages of shells vary from ΔR=−140 to +300 14 C years. Between 7300 and 5400 cal. yr BP, reservoir age and stable isotope values are highly variable and indicate mixing of marine water and brackish surface waters with hard water effects. After 5400 cal. yr BP, the ΔR values stabilise and show an increasingly marine environment, with 14 C reservoir ages close to 400 years (ΔR=0). After 2000 cal. yr BP, Kilen becomes brackish. Reservoir ages and stable isotope values are again highly variable and δ 13 C and C/N values are no longer correlated. Before 4000 cal. yr BP, δ 15 N values vary by only 1‰ and do not reflect the changes in the marine environment. A δ 15 N increase between 3500 and 2000 cal. yr BP signifies enhanced organic productivity or a change in agricultural practices.

The freshwater reservoir effect in radiocarbon dating

“…This division is supported by other proxies, as described in [33] and [31]. Figure 5 displays the development of R from c. 5400 BC to AD 700.…”

“…δ 13 C values are reported as VPDB. δ 15 N values and C/N ratios were measured at the same time and are discussed in detail in other publications [31][32][33].…”

“…Other sample types and measurements from this core, e.g. stable isotope measurements, are described in detail in [33,36].…”

“…It is therefore possible to study the influence of the freshwater reservoir effect on radiocarbon dates in the Limfjord over long time scales. Details on this study area are provided in [31,33,36,37].…”

“…These radiocarbon dates were obtained as part of different studies from the author's PhD project, all employing a variety of methods. See [17,18,[31][32][33] for details on the individual sub-projects. For this paper, the radiocarbon dating results of the different sub-projects are extracted and discussed in the context of other authors' studies on the FRE.…”